Light reactions: make ATP + NADPH for Calvin cycle
Review: Calvin Benson cycle fixes C and makes sugar
Rubisco - the enzyme that dominates carbon fixation
Carbon in often locked in highly oxidized forms, such as carbonate minerals and carbon dioxide gas. To be useful, as a building block of life, oxidized carbon must be “fixed” into organic forms, rich in carbon-carbon bonds
Most CO2 converted into biomass is fixed by rubisco
bridge between life and lifeless
creates organic C from CO2
Most abundant enzyme in the world
Large molecule : 16 polypeptides : 8 active sites
Rubisco: the clunky and slow carbon fixer
In spite of role, rubisco is remarkably inefficient
plant rubisco > cyanobacteria rubisco
Only 3-10 reactions per second
limits photosynthesis
20% error rate
gets worse at high temperatures
Hmmm…why is Rubisco so abundant?
Rubisco is a cheater!!!
Reacts with both O2 and CO2
O2 reaction called Photorespiration
uses ATP and NADPH to make CO2
totally wasteful!!!
Non-specific CO2 targeting related to evolution
Cyanobacteria evolved around 2.7 BYA
C3 photosynthesis how did we get here…
Evolution doesn’t always create the best solution
Photorespiration is completely wasteful
2x energy to produce the same amount of sugar than if Rubisco only reacted with CO2
Rubisco did evolve high affinity for CO2 (80x > O2)
compare to the composition of the atmosphere…
Rubisco substrate choice
Photorespiration impacts food security
Plants and humanity: Why does photorespiration matter?
Consumes 32% of ATP and 28% of NADPH in C3 leaves
under current conditions
Crop productivity improvement not sufficient to meet 2x food production by 2050
Photorespiration decreases US soybean and wheat yields by 36% and 20%
loss of 148 trillion potential calories
would feed 203 million people for a year
Plant evolution tied to atmospheric shifts in CO2:O2
Why has Rubisco not been replaced?
Evolution of enzymes difficult
trade-offs between activity and stability
Removing oxygen reaction breaks Rubisco
selection against modifying mutations
RubisCO → RubisC???
requires a decrease in fitness
plants are stuck with Rubisco
Natural selection increased affinity for CO2
makes it slow
Evolution tinkers with other parts of photosynthesis